In paper-making and cellulose machines and machines for similar purposes single-layer wires woven from metal wire or synthetic fibre threads of monofilaments or multifilaments are used for forming the sheets. On account of their poor wear resistance, metal wires are often replaced by wires made from synthetic fibre threads, so-called plastic fabrics. These plastic fabrics hitherto have been made almost exclusively as single-layer fabrics. However, fabrics of this type suffer from the disadvantage of being a great deal more stretchable than metal wires of comparative thickness gauge. For a long time the use of plastic fabrics therefore has been limited to the coarser wire assortment and to narrow and slow-moving paper-making machines. Although considerable improvement has been made during the last years, single-layer plastic fabrics have met with little success as concerns for instance broad and speedy newsprint paper machines and so-called tissue paper machines. Also in the case of broad liner, kraft and sack paper machines several attempts to use plastic fabrics have failed, although coarse and thus more stable fabrics were used.
So called double-layer plastic fabrics consisting of two layers of one yarn system and a second yarn system interconnecting the first two layers are, on account of their improved stability, more likely to succeed in all types of paper machines. This fact has also been documented by a large number of test runs. A double-layer fabric has for instance been in operation in a broad liner machine during six months as compared with eight to ten days for a metal wire. The problem met with in this kind of double-layer fabrics has hitherto been their tendency to mark the paper web to a larger extent than do single-layer ones. The reason for the increased marking tendency is that the two parallel layers of weft threads, and particularly the layer closest to the material to be formed, extend substantially straight and in plane inside the fabric, whereas the warp threads extend in a curved shape, tangent to the two outer planes. In the corresponding case in a single-layer cloth this disadvantage may easily be remedied by applying a load to the warp threads whereby their curvature is straightened while at the same time the weft threads become crimped. At a certain border value the wave crests are tangent to a common plane. If the single-layer fabric is fixed in this position, the web will contact both thread systems during sheet formation and the risk for marking decreases.
The geometric construction hitherto applied in the double-layer fabric has made a corresponding operation impossible, any straightening of the warp threads having instead resulted in the weft threads penetrating deeper down towards the centre of the fabric.
The present invention concerns a forming fabric for paper-making cellulose and similar machines, said fabric being made from a synthetic fibre material and comprising two layers of weft yarns and warp threads interconnecting said two layers. The purpose of the invention is to remedy the marking problems hitherto experienced in prior-art double-layer forming fabrics.